1. Field of the Invention
The present invention relates to a soundproof cover mounted to a power seat motor unit.
2. Description of the Related Art
A power seat mounted on a vehicle such as an automobile is provided with a power seat motor unit that electrically adjusts the front-rear position, the height, and the backrest angle of the seat (see Japanese Patent Application Publication No. 2015-209073 (JP 2015-209073 A)). The power seat motor unit is constituted of a combination of a plurality of components such as a motor and a gear. The seat can be moved in the front-rear direction and the height and the backrest angle of the seat can be varied as the gear is rotated by drive of the motor to rotate a drive shaft. In recent years, it has been desired to reduce vibration and noise in a cabin, and it has also been required to reduce a drive sound of the power seat motor unit.
A foam such as a polyurethane foam is often used as a sound absorption material for reducing noise. For example, Japanese Patent Application Publication No. 2015-069012 (JP 2015-069012 A) discloses a sound absorption cover made of a foam that contains a magnetic filler. In the sound absorption cover described in JP 2015-069012 A, the magnetic filler which has a high thermal conductivity is oriented in the thickness direction of the sound absorption cover. This allows not only a reduction of a motor sound, but also immediate discharge of heat generated by the motor via the magnetic filler.
When the power seat motor unit is driven, a radiated sound from the motor and a first gear and a structure borne sound from an attachment portion to a seat frame are generated. The rotational direction and the rotational speed of the motor are varied by a seat operation, and therefore the frequency of noise is varied. The sound absorption cover which uses a foam described in JP 2015-069012 A is effective in reducing noise at a high frequency from the motor. However, the sound absorption cover has little effect in reducing noise at a low frequency of 500 Hz or less such as the structure borne sound.
A dynamic damper is known as a device that reduces vibration (see Japanese Examined Patent Publication No. 6-100245 and Japanese Utility Model Application Publication No. 6-71938). In the dynamic damper, in general, a mass body is elastically supported on an attachment member, which is attached to a vibrating body as a vibration suppression target, via an elastic body such as rubber. When the vibrating body is vibrated at a particular frequency, the mass body and the elastic body constitute a mass-spring vibration system to resonate, and absorb and reduce vibration of the vibrating body. Although the dynamic damper is effective in reducing noise at a low frequency, the dynamic damper cannot reduce noise at a high frequency. In addition, the dynamic damper reduces noise in a narrow frequency band, and does not provide a sufficient noise reduction effect for an object that vibrates at a varied frequency such as in a case where the rotational direction and the rotational speed of the motor are varied.